NMR Study of Anion Dynamics in Solid KAlH4

1 H and  27 Al NMR is used to reveal the motions of AlH 4  anions in KAlH 4 . Line-narrowing from rotations and from translational diffusion is observed in the NMR of both nuclei. Unlike the anions in NaAlH 4  and LiAlH 4  that are not rotating on the NMR time scale at room temperature, the KAlH 4  anions are already rotating rapidly at 23 °C. Based on the onset of rotation-induced line narrowing, the  1 H  T 1  minimum, and the low- temperature hydrogen  T 1 ρ minimum associated with reorientations, the rotational activation energy  E rot,act  = 0.28 eV is determined. Similarly, we use the onset temperature of translational motion-induced line narrowing and the high-temperature  T 1ρ  minimum to determine the diffusion activation energy  E diff,act  = 0.70 eV. Lack of sharp structure in the first-order quadrupole pattern and the absence of second-order quadrupole structure in the  27 Al NMR data suggest asymmetry (η ≠ 0) and/or variations in the anion electric field gradients from structural disorder.